Increasingly far-reaching demands are placed in the automotive industry on the possibility of employing rational welding processes to fuse together sheet metal parts which later constitute the blanks in a pressing process. By such means, considerable material waste is avoided in that the blank may be given a configuration which corresponds to that of the finished part.
Those blanks which are under consideration here may many times consist of sheet metal materials of different thicknesses, and the sheet material may have different types of surface coatings. In such instance, it is then important that the surface coatings are damaged as little as possible, and that the effects of heat on the material are also kept within very narrow limits.
The prior art technology employed for welding in similar or identical practical applications to the present one has previously been resistance welding using copper wheels as electrodes (seam welding). Such a method is relatively simple and economical but does not guarantee high and uniform quality, since an overlap joint must be employed.
Laser welding has also previously been tried in practical applications of the type under consideration here. This welding method results in superior quality because of welding in a butt joint. However, productivity is low and investment costs are so high that this method can only be employed in exceptional cases.
Attempts have also been made in the art employing welding processes in which the welding heat is generated by resistive heating of the workpiece by using high frequency alternating current. In one such a welding process, the welding joint is heated simultaneously along its entire length, for which reason the workpieces must be aligned and advanced towards one another during the welding phase proper.